In many propel-vehicle applications that include hydraulic motors, it is desirable for the motor to have a parking brake or parking lock. Typically, brake packages which are used with hydraulic motors, and especially those brake packages used as integral brake packages with low-speed, high-torque (LSHT) gerotor motors, are of the “spring-applied, pressure-released” (SAPR) type as is now well known to those skilled in the art. In a typical SAPR braking assembly, the braking members (e.g., friction discs, etc.) are biased toward braking engagement by a spring arrangement, and are moved toward a brake-disengaged condition by hydraulic pressure, which may be internal case pressure, external “pilot” pressure from a system charge pump, or any other suitable source of pressure.
In most embodiments, a SAPR braking assembly utilizes a piston to apply or release a brake. The piston transfers force from a spring to a brake pack (e.g., a plurality of brake pads) to engage the brake assembly. The piston utilized to apply or release the brake is generally enclosed between a stationary housing and a rotating shaft, or between two stationary housings (see, e.g., U.S. Pat. No. 6,743,002). The inner and outer diameters of the piston are usually sealed by one or more seals, which may be dynamic or static. For instance, a seal on an outer diameter of the piston may be a static seal, while the seal on an inner diameter of the piston may be a dynamic seal or a static seal, depending on whether the piston is seated on a rotating shaft or a stationary housing. Due to the dimensional variations inherent in dynamic seals, these types of seals can be difficult and costly to design, and can wear out more quickly than static seals. Because of these design and wear constraints, dynamic seals are generally only suitable for small rotating shaft diameters. As shaft diameter increases, the pressure velocity (PV) factor for the seal increases, thus limiting the seal life and the size of a piston inner diameter.